The surveying of sub-surface geological formations by analyzing seismic energy, such as acoustic vibrations, has been done for many years. As is fundamental in such surveying, acoustic vibrations are generated at a source location in the area of interest and, after reflection (or, in some cases, refraction) mode conversion from sub-surface strata, are detected by receivers at numerous locations. The sensed vibration signals are typically recorded and subsequently analyzed by conventional computer equipment. The measured travel times of the seismic energy from the source to the receiver locations and the characteristics of the received energy each provide information concerning the sub-surface strata and interfaces therebetween, particularly the depth and location of potential hydrocarbon reservoirs. As is also well known in the art, such surveys are performed in both land and marine environments to determine the most suitable location for the drilling of a well for production of the hydrocarbon products.
Also as is well known, the receivers in such surveys not only detect the reflected seismic energy of interest, but also detect other vibrations which, for purposes of the survey, are considered as "noise". Such noise includes multiply-reflected vibrations, as well as other coherent, and also random, noise. A common surveying technique which provides enhancement of the "signal" portion of the detected energy relative to the noise is the use of multiple "fold" data. This technique activates the seismic source at a number of locations for receipt by multiple seismic detectors. The seismic energy detected over time is "gathered" by identifying those recorded traces corresponding to the same sub-surface reflection point (i.e., "common depth point" or "CDP") and summing these traces together in such a manner that the reflected signal of interest is enhanced relative to the noise. The number of traces gathered for a CDP, i.e., into a "bin", is commonly referred to as its "fold".
To generate a seismic survey in a desired sub-surface plane, receivers are arranged in a single line, and the location of the seismic source is collinear with or parallel to the line of receivers. This survey is commonly referred to as a "2-D" survey. Multiple fold data is obtained by moving the location of the source relative to the line of receivers so as to maintain a common depth point for multiple pairs of source/receiver locations. Conventional CDP gathering and other analysis of the recorded traces provides a survey of the sub-surface geology that is interpreted to be a single vertical plane beneath the shot line.
In order to survey the sub-surface strata over a surface area of the earth using this technique, however, multiple 2-D surveys must be taken from multiple parallel 2-D lines, with the resulting vertical plane surveys cumulatively analyzed to provide an estimated three-dimensional survey. Data acquisition by incrementally repeating 2-D survey lines is, of course, quite cumbersome. In addition, since the 2-D lines are parallel, rarely is seismic data received or analyzed which travels at known angles (i.e., azimuths) other than that of the parallel 2-D lines. Accordingly, even though data is detected over a two-dimensional surface of the earth, a true 3-D survey cannot be done since all data is received as if taken at a common azimuth.
Some prior techniques are directed to facilitating the acquisition of sufficient seismic data to generate a 3-D survey. A common one of such prior techniques is the so-called "swath" survey, where the receiver array consists of a number of relatively closely spaced parallel lines of receivers, for example spaced by a distance on the order of one-eighth of a mile apart, each line of receivers being several miles long. According to this method, the seismic source location moves in a direction along the length of the array (having positions within the array, outside of the array, or both), with the seismic signals detected by each receiver in the swath recorded and processed.
While the swath survey is commonly referred to as "3-D" due to the areal distribution of the receivers, true three-dimensional surveys are not generally obtained by this method. Firstly, the geometry of the swath necessarily provides a non-uniform azimuthal distribution of data, as most of the source-receiver paths are in nearly in the same direction, i.e., from close to the same azimuth, and are strictly determined by offset. As such, for a given source location significantly more data is obtained from within a narrow azimuthal range (e.g., on the order of 5.degree. or less) than at other angles, and the small amount of data that is acquired at different azimuths is necessarily limited to relatively short source/receiver offset distances. Secondly, the data processing techniques used with such swath surveys is conventionally limited to strictly 2-D analysis, by treating the data from varying azimuths as though it is at a common azimuth with the majority of the receivers in the swath array. Such analysis limits the resulting survey to providing multiple 2-D surveys in parallel vertical planes.
Another example of a so-called 3-D survey, using multiple parallel 2-D shot lines in a marine environment, is described in U.S Pat. No. 3,581,273. This reference shows a marine seismic survey method which uses a vessel having a towed streamer of receivers and towed sources which are not in line with the towed streamer. As described in column 6, lines 10 through 19, of this reference, three-dimensional records of the survey profiles are provided by multiple towed spreads of recording instruments, for example by towing three parallel lines of such spreads.
By way of further background, prior marine survey techniques for obtaining 3-D data are known which include parallel shot lines. One such method deploys bottom-fixed receivers, such as geophones, either in straight parallel lines or as a loop having two parallel sides of geophones and, usually, a "dead zone" around the curved portion.
Another 3-D marine survey using parallel shot lines is described in U.S. Pat. No. 3,906,352, where two vessels travel parallel to one another, each towing a streamer of hydrophones and a source. Each towed hydrophone array records shots from both the source towed by its own vessel, and from the source of the other vessel. As illustrated in FIG. 4 of this reference, the resulting survey includes the shot line of each vessel, and also shot lines between vessels.
By way of further background, U.S. Pat. No. 4,870,624 describes a method of obtaining a 3-D marine survey where cultural artifacts, such as existing drilling platforms, limit the navigability of the towing vessels. As described therein, stationary receivers are deployed in a line between two of the cultural objects, and a towed source is periodically activated as it travels in the survey area. The survey is accomplished by combining the results of the surveys of a number of sub-areas, according to a described method of transformation. Conventional 3-D migration is then applied to the transformed data.
Another prior method of acquiring some amount of 3-D marine survey data includes the towing of an areal arrangement of sources. U.S. Pat. No. 4,868,793, assigned to Atlantic Richfield Company, describes a method for towing an array of sources, and for controlling the timing of the firing thereof as used in a 3-D marine survey.
Full 3-D land- and marine surveys are described in U.S. Pat. No. 4,970,696, issued Nov. 13, 1990, assigned to Atlantic Richfield Company, and incorporated herein by this reference. In the land survey case described therein, seismic data of varying azimuths is acquired by arranging the receivers in multiple patterns, and moving the source location around the patch of multiple patterns. A similar survey is described in Crews, et al., "Applications of New Recording Systems to 3-D Survey Designs," Expanded Abstracts with Biographies, 1989 Technical Program, 59th Annual International SEG Meeting, Paper SA 1.6, (Society of Exploration Geophysicists, 1989), pp. 624-27, also incorporated herein by this reference. As described at column 3, line 66 through column 4, line 3 of said U.S. Pat. No. 4,970,696, this technique is applicable to marine surveys with the receiver patterns placed on the seafloor or suspended thereabove. According to another embodiment described therein, a marine seismic survey is obtained by the towing of an array of receivers (corresponding to a pattern in the land case) through the off-shore region of interest, where a separate source vessel travels around the towed array to provide source seismic energy at the appropriate locations.
In each of the full 3-D surveys described in said U.S. Pat. No. 4,970,696, seismic data is acquired at many azimuths (i.e., relative angles between source and receiver locations). This data provides for a true three-dimensional survey to be obtained, detecting sub-surface geological discontinuities which are at varying angles. In addition, other effects, such as near-surface effects, velocity changes, and the like may be characterized in the three-dimensional sense using this data. It should also be noted that the amount of data obtained (i.e., the fold) by such a true 3-D survey may be reduced, typically by a factor of from three to five, from that acquired according to prior 2-D surveys while maintaining the same degree of random noise attenuation. The theory explaining such fold reduction is described in Krey, "Attenuation of Random Noise by 2-D and 3-D- CDP Stacking and Kirchhoff Migration", Geophysical Prospecting 35 (1987), pp. 135-147, also incorporated herein by this reference.
The methods described in U.S. Pat. No. 4,970,696 provide accurate and thorough surveys which are fully three-dimensional, by acquiring data at varying azimuths. It has been observed, however, that such surveys also provide significant redundancy in the data acquired. In the towed array case particularly, inefficiency in the survey may result due to the time required for the vessel towing the receiver array to turn around and re-enter the survey area, such that the source and receiver arrays are in the proper relative position. The time during which vessels are traveling instead of generating and receiving seismic signals, can be significant for surveys of conventional size.
By way of further background, U.S. Pat. No. 4,933,912, issued Jun. 12, 1990, describes a three dimensional prospecting method which deploys an areal array of sources and receivers in the survey area. In the example shown relative to FIGS. 3 and 4, a 3-D land survey is shown where 45 stations of 23 lines are used for a 24 fold survey of an area 9,460 ft. by 4,840 ft. The reference further describes a method of data analysis where certain source-receiver pairs are selected for common midpoints, such that data of varying offsets and azimuths is acquired. The large number of receiver locations should be noted, particularly in considering the potential application of this prior technique to marine surveys.
It is therefore an object of this invention to provide a marine survey technique which acquires full 3-D seismic information at reduced surveying cost.
It is a further object of this invention to provide improved 3-D surveys due to improvements in noise reduction achieved by providing uniform spatial sampling.
It is a further object of this invention to provide improved 3-D and vertical seismic profile (VSP) surveys in which static corrections may be performed in three dimensions.
It is a further object of this invention to provide such a survey which reduces the number of receivers necessary, and hence reduces the redundant data acquired.
It is a further object of this invention to provide such a survey which efficiently utilizes the source vessel by increasing the fraction of time during which it can be providing seismic energy relative to the time required for travel between source line or segment locations.
It is a further object of this invention to provide such a survey which accurately acquires 3-D seismic information in areas with drilling rigs and other cultural obstructions.
It is a further object of this invention to provide such a survey which may be deployed in modular fashion, such that excess receivers need not be deployed beyond that required for the desired 3-D or VSP survey.
It is a further object of this invention to provide such a survey which allows for modular replacement of damaged receiver arrays, reducing the risk of liability and loss.
It is a further object of this invention to provide such a survey in which the depth of the receivers may be optimized.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.